...because it follows the "directories end with a / and files and webpages end with nothing" convention that seems pretty well-established.

None of these are actual directories, thought, correct? There's no such thing as finance/economy/index.php. But rather, finance/economy is itself the resource location. Keeping a trailing slash just for the illusion of a directory seems like unnecessary cruft to me.

DoubleDee said:

Would #4 even work as far as my mod_rewrite?

Your rewrite would likely have to change. But it'll probably work out better this way. You won't have to worry about naming collisions between sort names such as "by-date" and article slugs.